Lithium batteries, which in this application refers to both lithium ion batteries and batteries having a lithium electrode, must be sealed in a substantially moisture and oxygen impervious manner to avoid undesirable reactions with an electrolyte. Generally a "battery" will include one or a plurality of interconnected lithium "cells" with each cell usually being separately sealed. For applications such as cellular phones, a single cell is the usual power source.
Lithium batteries usually operate with non-aqueous electrolytes. The electrolyte can be a solid polymer bearing a dissociable lithium salt, but frequently the electrolyte is a liquid in which a lithium salt has been dissolved. The liquid commonly impregnates a porous polymer separator laminate which can be multilayered, or in some instances the porous separator is itself a lithium salt containing polymer laminate. The liquid in which the lithium salt is dissolved, can be ethylene carbonate, propylene carbonate or other alkyl radical bearing carbonates, or a similar organic compound, which has boiling point above 50.degree. C. and relatively low vapour pressure at room temperature. The electrolyte layer is located between the electrodes. Laminar lithium batteries are constructed of at least three layers, and the layered construction together with current carriers, is wrapped in a pliable polymer laminate container which thus forms a pouch.
As suggested above, one manner of sealing a lithium cell, and what is referred to herein as "sealed pouch cells" is to encase the cell in a plastic laminate covered foil pouch, the foil usually being of aluminium. Such an arrangement is shown in FIG. 1 which is a partially cut away perspective view of a sealed pouch cell. The cell, generally indicated by reference 10 has a positive electrode 12, a negative electrode 14, an electrolyte 16 and positive and negative "leads" or "current collectors" 18 and 20 respectively enclosed in a foil pouch 22. The leads 18 and 20 provide electrical communication between the positive and negative electrodes, 12 and 14 respectively, and an outside of the pouch 22.
The foil pouch 22 has an inner face 24 coated with a polymer laminate to both protect the foil from the electrolyte 16 and to prevent short circuiting between the positive electrode 12 and negative electrode 14 and the leads 18 and 20. Preferably the foil pouch 22 is also coated on its outer face with a polymer laminate.
The foil pouch 22 is heat sealed along three edges 26. A fourth edge 28 is typically formed by folding the foil from which the pouch is constructed and therefore does not require heat sealing. In order to ensure against the leads 18 and 20 coming into contact with the foil, particularly during the heat sealing operation, a separate insulating sleeve 30 is generally placed over each of the leads 18 and 20 in the vicinity of the edges 26 prior to heat sealing. The insulated sleeves 30 should form a seal and not act as an avenue for moisture ingress or electrolyte egress. Such a seal is referred to herein as "hermetically sealed".
For obvious reasons lithium batteries are sensitive to moisture and atmospheric corrosion, therefore the polymer pouches are sealed, usually by applying pressure and heat around the edges of the polymer laminate. Heat sealing a polymer laminate to another polymer laminate usually provides a very satisfactory bond, however the metallic leads or current collectors exiting in between the polymer layers may provide incomplete sealing, possibly resulting in seepage of the electrolyte liquid in spite of all efforts of to achieve a fast seal.
Although sealed pouch cells have numerous advantages associated with their construction and relatively thin profile, they are more easily damaged than cells encased in a more rigid enclosure. This poses significant problems as lithium batteries, particularly in rechargeable format, generally are used in expensive electronic equipment where damage through leakage is potentially very costly and totally unacceptable.
The leads 18 and 20 generally take the form of metal tabs, usually of aluminium, copper or nickel, which must be of sufficient thickness and size to carry a substantial amount of current without any appreciable voltage drop. This presents at least two problems in sealing the foil pouch around the area where the metal tabs exit. A first problem is that the metal tabs remove heat rapidly during the heat sealing and hence, provide uncertain sealing temperature control. Further, the metal tabs because of their thickness cause non-homogenous pressure which also leads to sealing uncertainty. As mentioned earlier, the lithium cells cannot tolerate ingress of moisture or oxygen or egress of electrolyte from the cell. Such leakage would lead to disaster for the cells and may also damage the equipment utilizing the battery or cell.
Further problems also arise through the diffusion of water molecules and oxygen through the polymer seal surface, which is that area of the polymer seal parallelled to the metal in the foil pouch. Water molecule as well as oxygen molecule diffusion through a metal is extremely low, however, water molecule and oxygen molecule diffusion through a polymer body is quite substantial. Hence, this destructive diffusion through the polymer sealing surface deteriorates the cell performance and leads to short life. Lithium cell applications whether for electric vehicles or laptop computers or cellular phones demand long life and cannot tolerate any cell failure through water and oxygen ingress. Also in a battery stack made with multiple cells, early failure in a single cell will lead to disaster for the complete battery pack. Because of these problems, lithium cells and batteries formed in foil pouches have not had any commercial success in spite of billions of dollars spent on its research and commercialization.
It is an object of this invention to provide a lithium battery which is resistant to mechanical damage and is at least doubly sealed against leakage.
It is a further object of the present invention to provide an enclosure for a lithium battery which is mechanically robust and forms an effective seal against moisture ingress and electrolyte egress.